The application of electron beam techniques to the fabrication of patterned substrates such as those used for microelectronic devices, has enabled a reduction in the pattern line widths for the device elements and also an increase in the pattern density. This has been accomplished mainly through perfection of the electron beam generating device (e.g, to increase the current density of the electron beam and/or to reduce the beam diameter) and through improving the materials used for defining the pattern on the substrate (i.e., resist materials).
In electron beam lithography, a substrate (e.g., silicon oxide, silicon, glass, quartz, or glass or quartz coated with a metal such as iron or chromium or the oxides, nitrides, and salts of these metals) is coated with polymer resist material. During patterning, the resist is exposed to a rastering or moving electron beam in sufficient dosage to change the solubility of the resist polymer to a particular solvent in the areas where the electron beam has interacted with the resist. To develop the electron-beam pattern, the solvent is used to dissolve away the parts of the resist that are soluble after exposure, while the parts that are not soluble after exposure remain on the substrate. The resulting pattern is used as a mask for forming a more permanent pattern on the substrate using a technique such as plating, chemical etching, ion-etching, ion-implantation, or photolithography.
As pattern density on these devices has increased, so has the need for methods to rapidly provide lithographic features with improved resolution. Proximity effects, which are due to electron forward scatter through the resist layer and backward scatter from the substrate, limit the line width and spacing of the lithographic features (see, e.g., T.H.P. Chang et al., Nanostructure Technology, IBM J. Res. Develop., Vol. 32, No. 4, pp. 462-492 (1988)). Reduction of proximity effects represents one means of improving resolution in electron beam lithography, thereby enabling the production of devices having higher pattern density.